1. Field of the Invention
This invention relates to a thin-walled, shirrable regenerated cellulose sausage casing having a reduced cellulose and plasticizer content and a process for preparing the casing.
2. Prior Art
Artificial sausage casings made entirely of regenerated cellulose have been widely used in the processing of frankfurters and related sausage products for a number of years. The basic process for the manufacture of casings of regenerated cellulose is known as the "viscose process", as for example described in U.S. Pat. No. 2,999,756, U.S. Pat. No. 2,999,757, and U.S. Pat. No. 3,835,113, and comprises extruding viscose, which is a solution of sodium cellulose xanthate, in caustic soda through an annular die into a coagulating bath to form a tubular casing.
The viscose solution is prepared by steeping chemically pure cellulose, typically of a wood or cotton source, in a concentrated caustic soda solution from which an alkali cellulose crumb is obtained. The alkali cellulose crumb is converted to cellulose xanthate crumb by reaction with carbon disulfide. After the reaction, the cellulose xanthate crumb is slurried with an aqueous dilute caustic soda solution in a proportion to yield from about 7 to 8 percent cellulose and a total alkalinity of from about 6 to 7 percent to form the viscose. The viscose usually has a degree of polymerization (D.P.) in the range of about 450-750.
The viscose solution is annularly extruded through an orifice in tubular form and substantially immediately coagulated and regenerated by passing it into an aqueous bath (Muller bath) which contains a mixture of sodium sulfate and sulfuric acid.
The salt/acid mixture is typically concurrently applied to both the outer and interior wall surface to effect simultaneous coagulation and decomposition of the xanthate thus regenerating the cellulose of the extruded product. The resulting wet gel tubing is washed and then plasticized by passing it through a water bath containing a plasticizer such as glycerine or a food acceptable polyol such as propylene glycol or diglycerol. The plasticized gel film is inflated under substantial air pressure and passed through dryer to remove a substantial portion of the water to produce the finished plasticized regenerated casing product.
Though regenerated casings manufactured by the aforesaid process can be utilized without further processing, a large portion of the commercial output is typically subjected to a very harsh and potentially damaging process termed "shirring", to provide lengths of casing in a convenient form for high speed stuffing. In such process, the finished casing is wound on reels and subsequently shirred on high speed shirring machines, such as those described in U.S. Pat. Nos. 2,010,626, 2,583,654, 2,722,714, 2,722,715, 2,723,201 and 3,451,827. In the shirring process, lengths of from 40-200 feet of casing are compacted into pleated strands of a few inches, e.g., 4-30 inches at a rate of 10 to 15 feet per second (ft./sec.). Such compacted strands allow easy handling of long lengths of casing and the meat packing industry considers it desirable to have the longest possible length of casing on a compacted strand. The shirr density, e.g., length of casing per inch of shirred casing is typically limited by the thickness of the casing and the shirring method.
The shirring process imposes several score pleats per foot of casing at extremely high rates of loading so that the casing must be flexible and strong enough to withstand such pleating without damage to the casing wall which later shows up as pinholes during high speed stuffing operations. The rapid extension of the casing during stuffing requires that the casing be especially strong and resistant to tearing. If even minor holes develop in the casing, the casing may split or break during stuffing with the disadvantageous loss of meat product.
After a casing is shirred, it is packaged and shipped to a meat packing house where an individual shirred strand is placed on a stuffing horn and a meat emulsion extruded to fill the casing to its fully extended length. The stuffing of the casing usually takes place within a few seconds with the result that the casing is extended from a shirred length of 8-27 inches to an extended length of 40-160 feet or more at a rate of 2-6 ft/sec.
It is therefore critical to the commercial utility of regenerated cellulose casings manufactured for subsequent shirring that the casing be sufficiently flexible, tear, and pinhole resistant to the harshness of shirring without damage and the resultant shirred strand must also be able to be readily deshirred during high speed stuffing operations without substantial breaking or pinholing. The shirred casing strand must also be of sufficient strength to withstand normal handling required for providing end closures in the casing and placement in high speed stuffing machines.
In order to obtain adequate flexibility and resistance to tear and pinholing, the manufacture of small diameter casing, e.g. up to about 4.0 centimeters in diameter, suitable for shirring has heretofore been commercially practiced under process conditions which assure casing wall thicknesses of about 1.0 mil and a plasticizer content of about 15 percent by weight or greater. Such thickness has been so accepted in commercial manufacture of casings to be shirred that most manufacturers consider a thickness of from 0.97 mil to 1.3 mil for small diameter shirrable casing as standard thickness. As a result of this industry evolved standard, coagulation and regeneration of the cellulose is commercially typically operated utilizing process conditions which maximize plant and casing efficiencies at such thickness. Thus, the coagulation and regeneration process is typically practiced in a single step with the dominant emphasis being the reduction of reaction time. Though both coagulation and regeneration can occur in a typical Muller bath over a wide temperature range, it is well known that the higher the temperature of the bath the faster regeneration occurs. Thus, the commercial practice of the prior art has evolved to maintenance of Muller bath temperatures at the 40.degree.-46.degree. C. range and most preferably at 42.degree. C. to maximize both reactions.
Similarly, the salt/acid content of the Muller bath can be maintained at widely divergent concentrations. The commercial practice of the prior art however is to optimize salt/acid concentration by balancing speed of reaction with chemical cost such that salt concentrations are maintained at their minimum efficient levels in a range from about 150 g/liter to about 250 g/liter and acid concentrations in a range from about 50 g/liter to about 135 g/liter. Typical of the prior art is U.S. Pat. No. 2,999,756 wherein the Muller bath of the examples is maintained at about 40.degree. C. and has a salt concentration of about 150 g/liter and an acid content of about 100 g/liter. In such bath both the coagulation rate and regeneration rate have been commercially and economically balanced to attain standard thickness casing.
Plasticizing of the regenerated casing product has heretofore been considered a necessary step in the conventional manufacture of small diameter regenerated cellulose sausage casing to impart sufficient flexibility and stretch characteristics to the casing to allow it to be shirred at high speeds and to prevent bursts during subsequent stuffing operations. Typically, an amount of plasticizer is incorporated in 1.0 mil standard regenerated cellulose sausage casing which is sufficient to provide adequate flexibility to the casing so that it can be safely subjected to high speed shirring without cracking or pinholing and thereafter will withstand the rigors of stuffing. In a standard 1.0 mil regenerated cellulosic sausage casing typically comprising about 65 percent cellulose, the amount of plasticizer required by the prior art to attain adequate levels of flexibility is between 15 percent to 25 percent of the total weight of the plasticized casing. Even containing such quantities of plasticizer, the standard 1.0 mil casing is also typically moistened to 16-20 percent moisture content during shirring to impart additional flexibility properties. Indeed, the necessity of plasticizers in shirrable casings is so imbedded in commercial practice that the author knows of no commercially available small diameter shirrable casing of standard thickness or less which contains less than about 15 percent plasticizer. At such standard thickness, a regenerated cellulosic casing containing from 15 percent to 25 percent plasticizer is seen in the prior art as having optimal flexibility and stretch characteristics for preventing bursts. Smaller thickness casings, down to 0.5 mil, using standard amounts of plasticizer, were speculated as having too little resistance to burst pressure and therefore not commercially practical. Greater thicknesses of small diameter casing, up to 1.5 mil were considered as merely a waste of materials and as adding characteristics which were undesirable to the consumer. Thus, the film wall of shirrable casings formed by the processes of the prior art and used for the processing of sausages generally commercially range in dry casing thickness from about 1.0 mil for small diameter shirrable casings to 3.8 mil for large diameter shirrable casings, e.g. greater than about 4.0 centimeters in diameter, or greater depending upon the circumference of the casing.
The amount of cellulose material per unit length of dried casing is conveniently indexed in the sausage casing art in terms of the weight of cellulose expressed in grams per 10 meters length (g/10 m) of a given casing width range or "Bone Dry Gauge" (BDG). The amount of cellulose material is also conveniently indexed by the weight in grams of cellulose per square meter (g/m.sup.2) and is termed "Basis Weight". The BDG of a casing will generally be varied by the manufacturer with the stuffing diameter of the casing as will the thickness. On the basis of comparative commercial tests, the BDG for a small diameter shirrable casing such as, for example, casing having a recommended stuffing diameter of 22.0-23.0 millimeter (mm) (Code 25), used for the processing of frankfurters, is typically 20 g/10 m and has a thickness of about 1.1 mil. The recommended stuffing diameter of regenerated cellulose casings used for small diameter type applications normally ranges from about Code 13 (13.0-15.5 mm) to about Code 52 ( 49.0-50.0 mm). For this small casing recommended stuffing diameter range the Basis Weight of the prior art casing typically ranges from about 24 for Code 13 casing to about 34 grams per square meter (g/m.sup.2) for Code 52 casing, the BDG ranges from about 10.2 for Code 13 casing to about 31.2 g/10 m for Code 52 casing and the dried wall thickness from about 0.97 to about 1.3 mil.
Though BDG is widely used as a convenient index of casing thickness, comparisons therebetween are generally only appropriate for casing walls having the same recommended stuffing diameter and composite structure. Thus, two casings of the same recommended stuffing diameter may have the same BDG but if the composite structure of one is different than the other, the thickness of the casings will generally be different. I have found that in a regenerated cellulose casing the morphology or microstructure of the regenerated cellulose casing wall is composed primarily of an outer skin portion on both the exterior and interior sides of the wall, the skin being characterized by large, poorly ordered amorphous regions and small or imperfect crystalline regions and an inner, less dense, core portion characterized by large crystalline regions separated by amorphous regions of fairly high order; the skin and core sections of the wall structure being readily distinguished from each other when samples of a torn dehydrated section are examined under a scanning electron microscope, the skin portion being tougher than the core and appearing more dense. Thus, the cross section of a regenerated cellulose casing wall appears to be comprised of two smaller crystalline outer skin laminates usually of higher density and a larger crystalline core of usually lower density such that by decreasing the amount of core material and increasing the amount of skin, the thickness of the casing can be significantly reduced while maintaining the BDG constant.
I have found that the wall of a conventional prepared Code 25 cellulose casing having a BDG of 20 g/10 m made under conventional commercial viscose process conditions will have a skin layer forming about 10 to less than 20 percent of the total casing wall cross-sectional area and typically averages about 15%. At such levels of skin content, I have found that standard thickness regenerated cellulosic casing typically has inadequate circumferential expansion and flexibility characteristics for utilization as a shirrable sausage casing without the addition of a plasticizer. In order to cure such defect the coagulated and regenerated gel casing is treated in the prior art by impregnating with from about 15 to about 25 percent by weight of a plasticizer, and then dried to form the final casing product.
Economic incentive exists to substantially reduce the Basis Weight and accordingly the BDG of the casing as well as to reduce or eliminate the expensive plasticizer component. Reduction of casing thickness is also desirable as it allows production of a shirred product having a higher shirr density. Prior efforts to reduce the Basis Weight of regenerated cellulose casing made under conventional viscose process conditions, however, resulted in casing products which were of insufficient strength for shirring and stuffing under modern commercial conditions. Attempts to manufacture casing of standard Basis Weight, but with elimination of the plasticizer, resulted in a shirred casing exhibiting a 50 percent tensile strength loss which encountered unacceptable high breakage during high speed commercial meat stuffing.